1. Field of Invention
The present invention relates to novel, highly active supported catalysts and a process for their preparation by pretreatment of an inert support material with a base and drying the so treated material to a specific residual moisture content prior to applying the catalytically active component, and to the use of the supported catalyst.
2. Discussion of Prior Art
Supported catalysts are understood as meaning compositions which contain one or more metals, as such or in the form of their compounds, in a very finely divided form on generally pre-shaped support materials and are employed for catalytic purposes. Catalysts of this type are commonly known and commercially available under very diverse designations, the active substances, in particular the noble metals, being applied in a very finely divided form, either as the metal or as a salt, to supports such as silicon dioxide, aluminium oxide, magnesium silicates and aluminium silicates, carbonates and others. Some specific examples of supported catalysts are Pd/.alpha.-Al.sub.2 O.sub.3 extrudates, Pd/V.sub.2 O.sub.5 /LiAl spinel spheres and platinum, palladium, rhodium, ruthenium or silver on aluminium silicate. Catalysts which have achieved very particular importance are those which contain the noble metals on supports with a small BET surface area, for example &lt;50 m.sup.2 /g. They are obtained by impregnating the corresponding support material with an aqueous solution of metal salts and/or noble metal salts and by subsequent reduction. The catalysts prepared in this way contain an irregular distribution of the active substance over the support. They are used in very diverse catalytic processes, for example in oxidation, polymerization, hydrogenation, dehydrogenation, epoxidation or vinylation or in hydrocracking. In the preparation of such catalysts a number of measures are required, which can be decisive for the subsequent industrial use of the catalysts, for example when the aim is to prevent the formation of undesired by-products. For example, the influence of the surface of the support material as a cause of the formation of by-products must be taken into account. Furthermore, it can arise that active substances present within a grain of the support, that is to say are present in the centre of the grain, either do not take part at all in the reaction to be catalyzed or participate in this reaction only to a minor extent.
For many catalytic processes, such as, for example, hydrogenation reactions, Al spinels, especially alkali metal aluminum or alkaline earth metal aluminum spinels, are used. When such alkali metal aluminum or alkaline earth metal aluminum spinels are prepared, differing residual amounts of water-soluble alkali or alkaline earth remain dispersed in the grain of the support, due to the process. Due to this inhomogeneity of the spinel, a variable deposition of the active substance in the individual grains of the support necessarily results. As a consequence of this, undesired differences in activity, selectivity and catalyst life can arise when these aluminum spinel catalysts are used industrially.
The active substances enriched within a grain of the support are in general lost for the reaction to be catalyzed. In the case of costly active substances, for example from the range of noble metal compounds, this circumstance results in a financial burden on the catalyst costs per unit of product prepared.
Attempts have repeatedly been made to prepare catalysts in which support materials with a small BET surface area contain the active substances only in a narrow, outer region of the grain of the support. Amongst the many attempts which have been made, the following measures for the preparation of supported catalysts on supports with a small BET surface area have, for example, been proposed:
Impregnation of the .alpha.-Al.sub.2 O.sub.3 support material with an aqueous solution of a base, partial drying of the support material treated in this way to a residual moisture content of 10 to 90% of the saturation amount and subsequent spraycoating with a solution containing a metal salt German (BRD) Offenlegungsschrift No. 1,944,933;
Pretreatment of a .gamma.-Al.sub.2 O.sub.3 support material with a base in order to precipitate metal compounds (for example Fe(OH).sub.3) on the surface of the support, the calcining process to convert .gamma.-Al.sub.2 O.sub.3 to .alpha.-Al.sub.2 O.sub.3 with an increase in the grain size and an increase in the pore size, promoted by the precipitated Fe(OH).sub.3, and subsequent impregnation with a solution of a noble metal salt according to German (BRD) Offenlegungsschrift No. 2,517,313;
Spraying of support materials with smooth surfaces, such as, for example, of carbonates, carbides and the like, with a colloidal PdO.H.sub.2 O solution prepared by neutralising a PdCl.sub.2 --HCl solution with NaOH, ripening over a period of up to 4 days and finally carrying out a further treatment with NaOH (U.S. Pat. No. 3,271,327);
Impregnation of a Al.sub.2 O.sub.3 support, which can contain up to 0.5% by weight of an alkali metal oxide and up to 10% by weight of a metal oxide in the form of Al spinel and is prepared by calcining Al.sub.2 O.sub.3 of the pseudoboehmite type, with a solution of a noble metal salt, the acidity of which is in a predefined ratio to the alkali metal content of the support German (BRD)Offenlegungsschrift No. 2,715,094;
Spraying of a calcined Al.sub.2 O.sub.3 with a concentrated, acidified solution of a Pd salt and subsequent calcining (U.S. Pat. No. 2,946,829);
Impregnation of supports which contain OH groups with a solution of noble metal-amine complex ions, which are obtained from the corresponding noble metal salts by the addition of aqueous NH.sub.4 OH, the support optionally being rendered hydrophobic beforehand German (BRD Offenlegungsschrift No. 2,317,536.
However, all of these processes and the catalysts thus obtainable have considerable disadvantages. In particular, these catalysts have a very high concentration of noble metal even on the surface of the support. Furthermore, differences exist between the individual catalyst particles in respect of the distribution of noble metal. A further disadvantage of the processes proposed for the preparation of the catalysts is that the support materials employed in general still have relatively large surface areas, as a result of which secondary reactions with the products can again be initiated. Furthermore, in many cases these catalysts have inadequate catalyst lives.
However, in the modern chemical industry there is a considerable demand for ever increasing amounts of supported catalysts, which, for reasons of cost, should contain as little noble metal as possible, enriched in a narrow annular zone, and should be distinguished by high selectivities and long catalyst lives.